According to a YouTube video, uploaded by Vietnamese Tinhte.vn, we see in the video bellow what is expected to be the upcoming Samsung Galaxy S III, at least feature-wise, but for sure not design-wise. As unveiled earlier, Samsung has given the Galaxy S III to carriers in fake-housings to keep the design secret. However, since carriers have to test and approve the device for their networks, it has to run on some of the core components and software which the later S III has to feature as well. Nevertheless, some of the hardware features of the testing device don't have to necessarily represent the final product, therefore it's not confirmed that the final S III will indeed sport the following features.
According to the video, the leaked Galaxy S III is running Android 4.0/Ice Cream Sandwich and sporting a 720p Super AMOLED Plus HD display, an 8 megapixel camera, NFC and software buttons as known from the Samsung Galaxy Nexus. It comes with 1 GB of RAM and 16 GB inbuilt flash memory which is expandable through microSD cards. All this is powered by a - most likely - Samsung Exynos Armv7 quad-core CPU at 1.4 GHz and a 2050 mAh battery.
Again, I wouldn't expect that the design is the final Galaxy S III design nor that all the components above, except the CPU are final. But chances are pretty high that in general, the device above represents what the final Galaxy S III, which Samsung is expected to announce May 3rd in London, might be.
However, since Tinhte.vn was quire right in the past and has often got its hands on devices prior launch, I would say that the video above isn't a fake but really something very close to what the Samsung Galaxy S III will be.
Cheers ~ Arne